内蒙古苏尼特左旗东苏A型花岗岩的锆石U-Pb年龄、地球化学特征及地质意义

位于苏尼特左旗东部的东苏二长花岗岩是索伦缝合带重要组成部分,具有高硅(SiO_2=69.76%~75.58%)、高碱且相对富钾(K2O+Na2O=6.99%~9.16%,K2O/Na2O=0.70~1.69)、相对富铝(铝饱和指数A/CNK=1.04~1.13)、高TFeO/MgO值(3.43~11.26)、贫镁(MgO=0.08%~0.50%)的特征;微量元素地球化学性质表现出相对富集轻稀土,Ba、Sr、Nb、Eu强烈负异常,Rb、Th、Ta等不相容元素富集的特征。主量和微量元素均表现出铝质A型花岗岩的特征。采用锆石LA-ICP-MS U-Pb法获得206Pb/238U值的加权平均年龄为221.5±0.81 Ma(MSDN值为0.57),表明该岩体为晚三叠世岩浆活动的产物。结合区域地质资料及地球化学信息可以判断该东苏花岗岩为后造山伸展阶段长英质地壳物质(变杂砂岩)部分熔融的产物。它的出现,标志着古亚洲洋演化的结束,索伦缝合带在晚三叠世的碰撞已达末期。

Geochronology, geochemistry and geological significance of Dongsu A-type granites in Sonid Left Banner, Inner Mongolia

The Dongsu granites from the Solonker suture zone are characterized by relatively high silicon (SiO₂=69.76%~75.58%), high alkali and potassium (K₂O+Na₂O=6.99%~9.16%, K₂O/Na₂O=0.70~1.69), high TFeO/MgO (3.43~11.26), and low MgO (0.08%~0.50%). Geochemical analyses show that the granites have relatively high LREE, obvious negative anomalies of Ba, Sr, Nb, Eu, and concentration of incompatible elements (Rb, Th, Ta). Major and trace elements indicate that the Dongsu pluton is of aluminous A-type granite. By LA-ICP-MS zircon U-Pb dating, a weighted mean ²⁰⁶Pb/²³⁸U age of 221.5±0.81 Ma (MSWD=0.57) is considered to be the formation age of the Dongsu granites which are the products of Late Triassic magmatic activities. Combined with geochemical information and regional geological data, it is concluded that the Dongsu granites were formed by partial melting of crust materials (meta-greywacke) at the stage of post-orogenic extension. Its intrusion marked the end of the evolution of the Paleo-Asian Ocean and the terminal of the collision of the Solonker suture zone in the Late Triassic.